ATOMIC ORBITALS

Atomic orbitals 

A Wave equation cannot tell us exactly where an electron is at any particular moment, or how fast it is moving: it does not permit us to plot a precise orbit about the nucleus. Instead, it tells us the probability of finding the electron at any particular place. 

The region in space where an electron is likely to be found is called an orbital. There are different kinds of orbitals, which have different sizes and different shapes, and which are disposed about the nucleus in specific ways. The particular kind of orbital that an electron occupies depends upon the energy of the electron. It is the shapes of these orbitals and their disposition with respect to each other that we are particularly interested in, since these determine, more precisely, can conveniently be thought of as determining the arrangement in space of the atoms of a molecule, and even help determine its chemical behavior. 

It is convenient to picture an electron as being smeared out to form a cloud. We might think of this cloud as a sort of blurred photograph of the rapidly moving electron. The shape of the cloud in the shape of the orbital. The cloud is not uniform but is densest in those regions where the probability of finding the electron is highest, that is, in those regions where the average negative charge, or electron density, is greatest. 

Let us see what the shapes of some of the atomic orbitals are. The orbital at the lowest energy level is called the 1s orbital. It is a sphere with its center at the nucleus of the atom, An orbital has no definite boundary since there is a probability, although a very small one, of finding the electron essentially separated from the atom-or even on some other atom! However, the probability decreases very rapidly beyond a certain distance from the nucleus, so that the distribution of charge is fairly well represented by the electron cloud in where the solid line encloses the region where the electron spends most (say 95%) of its time. 

At the next higher energy level there is the 2s orbital. This, too, is a sphere with its center at the atomic nucleus. It is-naturally-larger than the 1s orbital: the higher energy (lower stability) is due to the greater average distance between electron and nucleus, with the resulting decrease in electrostatic attraction. (Consider the work that must be done-the energy put into the system to move an electron away from the oppositely charged nucleus.) 

Next, there are three orbitals of equal energy called 2p orbitals, Each 2p orbital is dumbbell-shaped. It consists of two lobes with the atomic nucleus lying between them. The axis of each 2p orbital is perpendicular to the axes of the other two. They are differentiated by the names 2px, 2py, and 2pz, where the x, y, and z refer to the corresponding axes.